Most conventional motorized vehicles, such as the modern-day automobile, include a powertrain that operates to propel the vehicle and power the onboard vehicle electronics. The powertrain, which is sometimes referred to as a “drivetrain”, is generally comprised of an engine that delivers driving power to the vehicle's final drive system (e.g., rear differential, axle, and wheels) through a multi-speed power transmission. Automobiles have traditionally been powered solely by a reciprocating-piston type internal combustion engine (ICE) because of its ready availability and relatively inexpensive cost, light weight, and overall efficiency. Such engines include 4-stroke compression-ignited diesel engines and 4-stroke spark-ignited gasoline engines.
Hybrid vehicles, on the other hand, utilize alternative power sources to propel the vehicle, minimizing reliance on the engine for power, increasing overall fuel economy. A hybrid electric vehicle (HEV), for example, incorporates both electric energy and chemical energy, and converts the same into mechanical power to propel the vehicle and power the vehicle systems. The HEV generally employs one or more electric machines that operate individually or in concert with an internal combustion engine to propel the vehicle. Since hybrid vehicles can derive their power from sources other than the engine, engines in hybrid vehicles can be turned off while the vehicle is propelled by the alternative power source(s).
Series hybrid architectures, sometimes referred to as Range-Extended Electric Vehicles (REEVs), are generally characterized by an internal combustion engine in driving communication with an electric generator. The electric generator, in turn, provides power to one or more electric motors that operate to rotate the final drive members. In effect, there is no direct mechanical connection between the engine and the drive members in a series hybrid powertrain. The lack of a mechanical link between the engine and wheels allows the engine to be run at a constant and efficient rate—e.g., closer to the theoretical limit of 37%, rather than the normal average of 20%, even as vehicle speed changes. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine. This system may also allow the electric motor(s) to recover energy from slowing the vehicle and storing it in the battery by regenerative braking.
Parallel hybrid architectures are generally characterized by an internal combustion engine and one or more electric motor/generator assemblies, each of which has a direct mechanical coupling to the power transmission. Most parallel hybrid designs combine a large electrical generator and a motor into one unit, providing tractive power and replacing both the conventional starter motor and the alternator. One such parallel hybrid powertrain architecture comprises a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving power from the ICE, and an output member for delivering power from the transmission to the driveshaft. First and second motor/generators operate individually or in concert to rotate the transmission output shaft. The motor/generators are electrically connected to an energy storage device for interchanging electrical power between the storage device and the first and second motor/generators. A control unit is employed for regulating the electrical power interchange between the energy storage device and motor/generators, as well as the electrical power interchange between the first and second motor/generators.
Electrically variable transmissions (EVT) provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between the internal combustion engine and final drive, thus enabling relatively high transmission efficiency and the application of lower cost, less massive motor hardware. EVTs are also operable with engine operation that is mechanically independent from the final drive, in various mechanical/electrical split contributions, thereby enabling high-torque continuously-variable speed ratios, electrically dominated launches, regenerative braking, engine-off idling, and two-mode operation.
An EVT can use what is commonly known as “differential gearing” to achieve continuously variable torque and speed ratios between input and output without sending all power through the variable elements. The EVT can utilize the differential gearing to send a fraction of its transmitted power through the electric motor/generator(s). The remainder of its power is sent through another, parallel path that is mechanical and direct (i.e., “fixed ratio”), or alternatively selectable. One form of differential gearing is the epicyclic planetary gear arrangement. Planetary gearing offers the advantage of compactness and different torque and speed ratios among all members of the planetary gearing subset. However, it is possible to design a power split transmission without planetary gears, for example, as by using bevel gears or other differential gearing.
Traditionally, a number of hydraulically actuated torque establishing devices, such as clutches and brakes (the term “clutch” used hereinafter to refer to both clutches and brakes), are selectively engageable to activate the aforementioned gear elements for establishing desired forward and reverse speed ratios between the transmission's input and output shafts. Shifting from one speed ratio to another is performed in response to engine throttle and vehicle speed, and generally involves releasing one or more “off-going” clutches associated with the current or attained speed ratio, and applying one or more “on-coming” clutches associated with the desired or commanded speed ratio. The speed ratio is generally defined as the transmission input speed divided by the transmission output speed. Thus, a low gear range has a high speed ratio, whereas a high gear range has a lower speed ratio.
In general, ratio changes in a transmission should be performed such that torque disturbances are minimized, and the shifts are “smooth” and “unobjectionable”. Additionally, release and application of clutches should be performed in a manner which consumes the least amount of energy, and does not negatively impact durability of the clutches. A major factor affecting these considerations is the torque at the clutch being controlled, which may vary significantly in accordance with such performance demands as acceleration and vehicle loading. In certain EVTs, shift torque reductions can be accomplished by a zero, or close to zero, torque condition at the clutches at the time of application or release, which condition follows substantially zero slip across the clutch.
The range change in some EVTs is controlled through a two-clutch synchronization and release process. Therein, a first clutch associated with a currently active range is carrying torque in an applied state, while a second clutch associated with a currently inactive second range is carrying no torque in a released state. Shifting from a first range to a second range is accomplished by controlling the unapplied clutch to zero slip speed, and applying the clutch thereby placing the EVT in a two clutch application state. During the two-clutch application state, the engine is directly mechanically coupled to the transmission output. The two clutch application state is then exited, and the second range effected by the release of the first clutch during control of the first clutch to zero slip speed.
Conventional EVTs are designed to operate in both fixed gear (FG) modes and electrically variable (EVT) modes through the controlled activation of the torque-transfer clutches described above, typically employing a hydraulic control circuit to regulate clutch actuation. When operating in a fixed gear mode, the rotational speed of the transmission output member is a fixed ratio of rotational speed of the input member from the engine, depending upon the selected arrangement of the aforementioned differential gearing subsets. When operating in an EVT mode, the rotational speed of the transmission output member is variable, based upon operating speeds of the aforementioned electrical motor/generators, which can be connected to the transmission output via actuation of a clutch, or by direct connection.
Many electric hybrid transmission configurations are capable of vehicle propulsion with the engine stopped. Powertrain functionality must remain functional during engine-off operation to meet operator demands and ever-changing operating conditions. For optimal efficiency and performance, the transmission should be able to shift between different EVT modes based on driver demands and vehicle speed. Typically, shifting from one EVT mode to another EVT mode in a synchronous manner is impossible when the engine is stopped because the intermediate state is a fixed gear.